1. Technical Field
The present invention relates to bicycle parts, and more particularly, to an expander nut for being assembled atop a stem of a bicycle so as to pull the stem downward to preload a set of head parts of the bicycle.
2. Description of Related Art
Various assembling parts for assembling a stem, a set of head parts, a head tube and a steering tube of a bicycle have been developed to date. One instance is U.S. Pat. No. 7,204,502, titled “Steering Bearing Assembly for Vehicle”. In the following description related to this prior patent, all recited elements are referred to the numerals indicating the corresponding elements shown in the drawing sheet of the published Patent as can be obtained from the USPTO Web Patent Databases. This prior patent disclosed a head tube 30 accommodating a steering tube 23 and having a top bearing 40 and a bottom bearing 45 assembled to the top and bottom ends thereof. A handlebar stem tube 62 of a handlebar assembly is mounted atop the steering tube and preloads the top bearing 40 at the top end the head tube 30. A bolt 65 is installed in the top of the steering tube 23 to exert a downward force to the handlebar stem tube 62 so that the handlebar stem tube 62 presses downward and thus position the top bearing 40. Finally the handlebar stem tube 62 is tightened onto the steering tube 23 of the bicycle so that the stem, the head parts, the head tube and the steering tube are well assembled.
Referring to FIG. 1 provided in the present application, the bolt 65 recited in the aforementioned prior patent is composed of a start nut 651, a screw 652 and a top cap 653. The screw 652 centrally passing through the top cap 653 and getting engaged with a threaded hole 654 of the start nut 651. The star nut has two discs of hardened steel each with a plurality of sharp tabs 655. The start nut 651 is placed into the steering tube so that the sharp tabs 655 engage on the internal bore of the steering tube. Due to the configuration of the discs, the start nut 651 is allowed to move inward the steering tube and retained from leaving the steering tube. The top cap 653 presses on the top of the stem so that when the screw 652 is tightened, the top cap 653 is pushed downward and in turn preloads the top bearing 40.
In the past when steering tubes were made from steel the star nut 651 was acceptable. Now steering tubes are made from aluminum alloy or even from carbon composite materials and so the damage that a star nut can cause when it is hammered in position is unacceptable and potentially dangerous. Besides, once the star nut 651 is assembled it cannot be removed without causing further damage and destructive deformation. Thus, the traditional device is unfavorable to stem replacement or head-parts preload adjustment.
Another instance is U.S. Pat. No. 5,737,974, titled “Fastening Assembly of Bicycle stem”. In the following description related to this prior patent, all recited elements are referred to the numerals indicating the corresponding elements shown in the drawing sheet of the published Patent as can be obtained from the USPTO Web Patent Databases. In the prior patent, an assembly composed of a bolt 2, a stop cover 12, a spring torsion 6, an arresting tube 4, and a tightening head 3, gives a upright tube 1 a downward force. The spring torsion 6 is settled between the stop cover 12 and the arresting tube 4. The bolt 2 passes through the stop cover 12 and the arresting tube 4 to engage with a threaded hole of the tightening head 3. The arresting tube 4 is configured to have outward expandability. The tightening head 3 is settled in the arresting tube 4 and has a tapered outer periphery. When the bolt 2 is tightened, the tapered outer periphery of the tightening head 3 expands the arresting tube 4 to abut against the inner surface of the steering tube 7. The stop cover 12 pressed by the bolt 2 exerts the downward force to the upright tube 1, so as to fasten the head parts 9.
Though the spring torsion 6 is configured to compress when the arresting tube 4 is expanded, it can sometimes resile before the arresting tube 4 wedges the steering tube and thus causes the whole assembly to fail in preloading the upright tube 1. Besides, as the arresting tube 4 is substantially a tube with slots formed thereon and the tightening head 3 uses the tapered outer periphery to expand the arresting tube 4 from inside and support the expanded arresting tube 4 to tightly wedge the inner surface of the steering tube 7, gaps and concentrated point load between the arresting tube 4 and the steering tube 7 out of uneven expansion tend to happen and damage the steering tube.